One common theme could be the clash and then the melding of their different cultures. T'Challa from Wakanda and Tony Stark, the high - tech billionaire. Another theme might be their shared sense of responsibility and how that affects their relationship.
Common themes include the discovery of each other. Since they come from such different worlds, there's the aspect of getting to know one another's values, beliefs, and ways of life. Another is the theme of loyalty. How loyal are they to each other in the face of various temptations and difficulties? And of course, the theme of love against all odds. Their relationship might be frowned upon by some in the fanfiction universes, and seeing how they overcome those obstacles to be together is a popular theme.